The invention relates to a decorative foil and a method of marking the decorative foil.
Such decorative foils are used for safeguarding and protecting markings and inscriptions, photographs and other indicia on documents such as passes, identity cards, credit cards, banknotes etc, or for packaging valuable articles.
U.S. Pat. No. 5,331,443 describes a method in which a freshly embossed and light-diffracting relief structure covered by a reflection layer can be individualised by applying marking thereto. The reflection layer is locally removed by means of laser beams and then covered with an adhesive layer. The disadvantage of this method is that the individual marking has to be applied prior to finishing of the layer composite structure by application of the adhesive layer.
It is also known (U.S. Pat. No. 2,590,906) that a dielectric coated with a metal forms an interference filter which in daylight reflects brilliant interference colors. U.S. Pat. No. 3,858,977 describes the suitability of multi-layer interference filters as an authenticity feature for documents. In accordance with U.S. Pat. No. 3,338,730 structured surfaces which do not diffract daylight are covered with such interference filters in order to obtain an attractive packaging material which lights up in the interference colors.
WO 98/19869 describes a method of perforating documents by means of focussed laser light. The density of the holes produced in the document and the diameter of the holes are determined by a pattern of an original. For that purpose an optical sensor senses the original and converts the recognised gray tones of the original into density and diameter of the holes which are burnt in the document. In that way it is possible to produce images, portraits or letter images which can be seen when looking through the document and which are practically impossible to counterfeit. Uses of that method are described in the article entitled ‘Application of laser technology to introduce security features on security documents in order to reduce counterfeiting’ by W Hospel in Proceedings of SPIE, Vol 3314, 28–30 Jan., 1998, pages 254–259.
The article ‘Search for effective document security by “inventioneering”’ by J D Brongers in Proceedings of SPIE, Vol 3314, January, 1998, pages 29–38 describes that intensive, finely focussed laser light is used to remove color layers on the surface of a substrate in a predetermined pattern from a print image without damaging the substrate itself.
EP 0 201 323 B1 describes the layer structure of plastic foils which include embossed holograms with transparent reflection layers. The materials suitable for the layer structure and the reflection layer are summarised in that document.
Transparent dielectrics with a very high refractive index for increasing the reflection capability in respect of diffraction structures are known from WO 99/47983.